The Hubble Space Telescope Key Project on the Extragalactic Distance Scale

The main goal of this HST Key Project is to determine the Hubble Constant, H0, to an accuracy of 10%. This goal is achieved by

(1) refining the cosmic distance scale by observing 18 spiral galaxies within 20 Mpc, searching for and measuring the periods and magnitudes of Cepheid variables, and

(2) calibrating the secondary distance indicators such as the Tully-Fisher relation for spirals, fundamental plane method for ellipticals, surface brightness fluctuation method and Type Ia supernovae.

The project was started in summer 1984. It has now grown into a team of 28 members. A list of publications, Cepheid data, finding charts can be found at A good review article which describes the strategy of the Key Project is found in Kennicutt, Freedman and Mould, AJ, 110, 1476. The HST observations of galaxies finished in January 1998. A composite of I-band period-luminosity relations of most of the galaxies is found here:

I have also written a couple of popular-science magazine articles in Japan. The English versions are:

The Hubble Constant and the Hubble Space Telescope Published in Suuri-Kagaku (Mathematical Science) in Japan, July 1997.

Observing the Expansion of the Universe using the Hubble Space Telescope Published in Illume magazine, October 1997.

Calibration of the Tully-Fisher Relations

The calibration of the Tully-Fisher (TF) relations and determining the H0 using these relations comprises the core of the Key Project. I have been leading an effort of calibrate both optical and IR TF relations and applying them to distant cluster samples to determine the H0. (A preprint will be ready *soon*, hopefully by the end of May!!)

John Huchra and Lucas Macri at Harvard-Smithsonian Center for Astrophysics have obtained BVRI images of all of the Key Project galaxies, and Lucas has recently finished the reduction of these data, including performing surface photometry. The figure below compares I-band and H-band TF relations before (left) and after 1993, showing how much the HST observations helped in refining the zero points of distance indicators. The H-band photometry data are from the survey by Aaronson, Huchra and Mould (cf. Aaronson et al. 1980, ApJ).

Red solid circles represent four galaxies (M31, M33, NGC 2403 and M81) whose Cepheid distance measurements were available before the HST re-furbishment. Additional 15 galaxies were included in "1999" figures. Note how the zero point became brighter by ~0.3 mag, which translates to a direct decrease in the value of H0 by 15%.

Hubble Constant

We applied the I-band TF relation to the clusters of galaxies presented in Giovanelli et al. (1997 ApJ). The Hubble diagram is shown below. Averaging the H0's derived for five clusters with velocities 5000 km/s or larger, we obtain H0=73 km/s/Mpc.